This invention relates to mobile scaffolds adapted to roll on caster wheels from one location to another, the caster wheels being vertically adjustable so that the scaffold may be leveled at the point of use.
Generally, such scaffolds have a pair of spaced-apart vertical frames supporting an elevated work station therebetween and vertically adjustable caster wheels at the corners of the scaffold. One example of such a scaffold is a mobile ladder unit, such as shown in U.S. Pat. No. 2,964,122 or U.S. Pat. No. 3,327,810 wherein a vertical ladder is supported between two wheeled side frames. Another example of such a scaffold is a platform scaffold wherein an elevated horizontal work platform is supported by two wheeled end frames, such as shown in U.S. Pat. No. 3,454,131.
An important factor in the design of scaffolds is the height-to-width ratio, i.e., the ratio of the height of the work area to the minimum base width of the scaffold. In order that the scaffold have sufficient stability to prevent tipping over, it is usually recommended that such ratio not exceed 3 to 1.
The maximum width of the scaffold is oftentimes dictated by the necessity that the scaffold be movable through relatively narrow standard doors or along narrow aisles or corridors. In order to use such a scaffold in a high-ceiling location, the ladder or platform must be raised to a height sufficient to enable the desired work to be carried out. If this height exceeds the 3 to 1 ratio, conventional practice has been to use outriggers or outboard supports to widen the base of the scaffold and thereby reduce the height-to-width ratio to a safe amount.
Mobile scaffolds also must be provided with vertically adjustable legs in order to provide a solid horizontal base for the scaffold regardless of the uneven slope, or stepped nature of the floor or ground where the scaffold is positioned. This requirement of vertical adjustability also applies to any outrigger or outboard support attached to the scaffold to increase the base width thereof.
An outrigger is conventionally a diagonally disposed prop or brace clamped onto the scaffold to extend outwardly and downwardly therefrom into engagement with the ground. The above-mentioned U.S. Pat. Nos. 2,964,122 and 3,327,810 illustrate examples of such outriggers, two being used, one on each side of the scaffold. In the case of a platform scaffold, of the type shown in U.S. Pat. No. 3,454,131, such outriggers would be used at each of the four corners of the scaffold.
If the diagonal bracing member is telescopic, e.g., as in U.S. Pat. No. 3,327,810, it has the capability of vertical adjustment, but with the corresponding disadvantage that outward distance of the outrigger foot from the scaffold will vary considerably, depending on the slope of the ground, and interference with vertical obstacles may result. Another disadvantage of a conventional outrigger is that if it has no telescopic diagonal adjustment, i.e., as in U.S. Pat. No. 2,964,122, or a limited telescopic adjustment, it must rely for vertical adjustability on being frictionally clamped to a vertical member of the scaffold. As a consequence, a suitable range of clamping positions may not be available because of cross tubes or the like secured to the vertical member.
A further disadvantage of conventional outriggers is that it is not feasible to provide them with caster wheels. Therefore, all conventional outrigger designs are provided with a "crutch-tip" or "ladder shoe" or other stub end which contacts the floor. In consequence, there is the necessity for readjusting and/or reclamping each outrigger (as well as adjusting the various casters) each time the scaffold is repositioned.
A rolling scaffold using outriggers, both in being moved and in working position, carries its full weight on the main scaffold casters. The outriggers do not function to provide vertical support to the scaffold.
A conventional outboard support is a vertically oriented rectangular frame having one side thereof attached to a vertical corner of the scaffold and a downwardly depending vertically adjustable ground-engaging member at its other side. As with outriggers, the outboard support is clamped or otherwise attached to the scaffold frame at two vertically spaced points. Oftentimes the outboard support is provided with a vertically adjustable leg-and-caster arrangement and may therefore be rollable with the scaffold on a smooth floor. However, at each work position of the scaffold, each scaffold leg must be adjusted and each scaffold caster must be locked. Likewise, each outboard support leg must be adjusted and each outboard support caster must be locked. Thus, again, conventional outboard supports double the number of caster-lockings and caster-unlockings and leg adjustments that must be made at each working position to achieve a firm base for the scaffold. Even then, it is not possible, under normal use, to be sure that the weight of the scaffold is uniformly distributed on the various scaffold and outboard support legs.
Outboard supports are neither designed nor available to serve as the sole vertical supports for the weight of the scaffold and workman thereon and therefore the maximum amount of stability that could be available from this outward engagement with the ground cannot be realized. The reason for this lack of capability of outboard supports to act as the sole vertical support for the scaffold is that if the lower fastening of the outboard support (in tension) should fail, the scaffold would tip over. Likewise, if the upper fastening (in compression) should fail, the scaffold would fall over.
A further disadvantage of a conventional outboard support is the problem of its pivotal orientation to the scaffold. If it must be clamped to the scaffold at a certain horizontal angle relative thereto, it will often interfere with a nearby obstacle. If it is pivotally adjustable, a separate brace is necessary to fasten the support to the scaffold to hold it in that pivoted position since the limited vertical force exerted between the support caster and the ground will not usually resist pivotal movement of the support if it is accidentally hit.
The only conventional alternative to the use of outriggers or outboard supports has been to supply users with special wider-base frames, to be installed when a wider scaffold is needed. Although this does enable higher platforms to be used and offers the economy of using only four adjustable legs and casters on a four-corner scaffold, it is an impracticability for a mobile scaffold which must be rolled through doorways and narrow aisles to be economically useful.
As a consequence of the limitations of outriggers, outboard supports and wide-base frames there has been an unfulfilled need for a mobile scaffold which can be easily and safely narrowed for rolling movement and widened for use and which requires no more adjustment than would be necessary for a scaffold without outward support.
It is the main object of the present invention to provide such a scaffold.